The present invention refers to a method of and apparatus for making a bottom of a tubular section for formation of a bag or sack.
In general, sacks or bags are made of flat tubular sections which are provided at their respect ends with serrations for defining the bottom edges. Subsequently, the ends for forming the bottom are partially opened by suction means, whereupon corner flaps are formed, and bottom score lines are introduced into the opened bottom for formation of parallel outer bottom edges so that the side bottom flaps are foldable along the score lines for closing the bottom. Bottoms of this type are referred to as cross bottoms or also block bottoms.
Conventionally, block bottom bags or block bottom sacks are made by so-called cylinder machines which are dimensioned in such a manner that the tubular sections are conveyed longitudinally in conveying direction, i.e. the longitudinal edges of the tubular sections extend in conveying direction. With machines of this type, block bottoms can be formed only on the leading ends of the tubular sections. In general, block bottom bags or block bottom sacks are made of paper. In particular in connection with block bottom sacks, several paper layers may be used, with the material layers of the folded bottom being joined together through gluing.
A particular advantage of block bottom bags or block bottom sacks is the precise parallelepiped or brick-shaped configuration of the bag when being filled, which is particularly advantageous for a subsequent stacking and palletizing. This configuration is accomplished however only when forming both ends of the bag or sack with such block bottoms. To date, a mechanized production is, however, not possible.
For that reason, so-called cross bottom valve sacks are widely used which differ from block bottom bags by their lack of side folds. Thus, a cross bottom bag will not assume a precise brick-shaped configuration when being filled. After opening the tubular section ends, a so-called open bottom is created, with the corner flaps being formed along the longitudinal edges. Cross bottom sacks have corner flaps describing a rectangular triangle, and the diagonal bottom edges extending to the bottom center at an angle of 45.degree. intersect the longitudinal edge of the sack. In block bottom bags, the material layers of the side folds are positioned between both diagonal bottom edges so that the corner flaps are of trapezoid configuration. Due to the material layers of the side folds, the height of the open bottom in a block bottom is greater by twice the depth of the side folds than the height in cross bottoms.
As the formation of the open bottom is technically more complicated in the case of block bottoms, block bottom sacks or block bottom bags are transported longitudinally in conveying direction during production. The capacity of such a machine is significantly lower compared to a machine in which the bags are transported transversely to the conveying direction. The reason is that at same power the throughput rate of the tubular sections in a transport longitudinally in conveying direction is much greater because the length of the tubular sections is generally greatly exceeds the widths thereof.
Cross bottom sacks are made by machines in which the sacks are conveyed transversely to the conveying direction and the opening of the tube ends to form the open bottom is carried out by two opening tools which are rotatably driven in opposition to each other and rotate about vertical axes, or axes which are inclined relative to the vertical. The speed of the opening tools is irregular and is the greatest at the moment of entering the partially opened end. This technique has proven effective only in conjunction with making cross bottom sacks. In connection with making block bottom sacks, this technique is unsuitable because the opening tools retract prematurely from the ends of the tubular section, since, as stated above, the height of the open bottom in block bottoms is significantly greater than the height in cross bottoms. In block bottoms, the fraction of material being shaped is also much greater than in cross bottoms.